The invention relates to the art of papermaking, and particularly to a method for conditioning fibrous webs such as paper and paperboard webs on a papermaking machine after the web is dried to improve the properties of the web.
The conventional process of papermaking involves formation of a web of fibers on a papermaking machine using a moving porous foraminous support wherein water is drained from a dilute slurry of fibers deposited on the support with further water removal from the web in a press roll section and final removal of water in a dryer section of the machine. In a typical papermaking process, the fibrous web from the press roll section contains about 32 to 45 wt. % solids. The solids may include wood pulp and/or synthetic fibers along with various additives such as sizing agents, binders, fillers, pigments and the like. The wet web is then passed through a series of internally heated rolls or steam-filled cylinders in the dryer section where the web is dried to about 94% to about 99% solids content by weight. The number of drying cylinders in the dryer section is determined by the amount of water to be evaporated based on a typical evaporation rate of about 3 to about 5 pounds per hour per square foot of total dryer surface.
In the dryer section of the paper machine, water is removed from the web mainly by evaporation. Typically, the wet web is alternately contacted on its opposite sides in serpentine fashion with a series of heated co-rotating cylinders to heat the web to a temperature sufficient to evaporate water from the web to the desired dryness.
Once dried, the paper or paperboard is often further treated to improve various properties such as smoothness, gloss, wet strength and folding endurance. This further treatment may include adjusting the moisture content of the dried-web, densification on high pressure rolls, calendering and/or heat-treating the product.
Various problems have persisted in the drying and calendering of paper webs on large, high-capacity paper machines. For example, drying and calendering of the products remains a high energy, capital intensive operation. Hence, the industry is challenged to develop newer and more energy efficient drying and calendering techniques. Such techniques include high-intensity drying methods where high temperatures and mechanical pressure is applied to the web during drying. Examples of currently used high-intensity drying techniques include press drying, impulse drying, and thermal/vacuum drying. However, the use of high temperature dryers and/or impulse dryers has led to additional problems such as delamination of multiply board products.
Furthermore, in the presently used drying and calendering processes, the paper may shrink in width by as much as 5 to 6% which can lead to a significant reduction in the overall production rate, and adversely affect product quality.
Accordingly, even with modern, state-of-the-art drying and calendering techniques, there remains a need to further improve the drying and calendering of paper and paperboard products to reduce energy costs and limit paper shrinkage without adversely affecting the physical properties of the finished product.
Uneven drying and streaking are other problems which have persisted in production of paper and paperboard webs. The weight and moisture irregularity of the fiber web before drying and calendering, irregularities in the heat transfer from the cylinders, edge effects and variations in the ventilation of the papermaking machine all tend to cause nonuniform drying in the cross-machine direction of the product. Such nonuniformity of drying can lead to further adverse effects on paper quality and increased waste.
U.S. Pat. No. 4,378,639 to Walker and U.S. Pat. No. 4,474,643 to Lindblad propose a solution to the problem of uneven drying across the width of the web by periodically spraying water on the web in selected areas across width of the web where low moisture or dry streaks have been detected. Because the water sprays are intermittent and used only when necessary to prevent streaks, such techniques do not effectively increase the drying rate of the web and can introduce nonuniformity in the web surface properties. These and other such approaches also present problems in that the spray nozzles can drip onto the web or otherwise tend to wet the paper in spots or unevenly, resulting in poor efficiency and surface discontinuities in the rewetting, drying and calendering steps, as well as other operational problems.
It is therefore an object of the invention is to improve the efficiency, uniformity and product quality of drying and/or calendering steps in a papermaking process.
A further object of the invention is to provide a more effective method for conditioning paper and paperboard products prior to rewetting the products.
Yet another object of the invention is to increase the drying efficiency of a papermaking process.
Another object of the invention is to provide a method for conditioning a paper or paperboard product for calendering which reduces operational problems associated with prior methods, and improves surface finishing.
Another object of the invention is to provide an efficient means of cross-directional moisture profiling of a paper or paperboard product on a papermaking machine.
With regard to the foregoing and other objects, the present invention provides, in accordance with its more general aspects, a method for treating an elongate moving web which comprises conditioning the web by applying a flow of moistened gas to a surface of the moving web across its width and along at least a portion of its length from a plurality of substantially overlapping flow zones wherein the flow in each zone is sufficient to create a combination of suction and pressure forces on the surface of the web to promote convective heat transfer and thereby decrease its surface temperature.
As used herein, xe2x80x9cmoistened gasxe2x80x9d means a carrier or motive gas, such as air, which has an absolute humidity of 0.01 or higher. The state of the water in the moistened gas may be vapor, or more preferably primarily liquid in the form of a relatively fine dispersion of small droplets such as a mist combined with evaporated water in the form of gas. As will be described in greater detail hereinafter, it is a feature of the invention that the water droplets are, by virtue of the flow regime used to deliver the moistened gas, propelled against the surface of the web so as to make contact therewith in a relatively even and highly dispersed manner to thereby achieve uniform and rapid cooling and moisturizing of the web.
In one embodiment, the method comprises treating a fibrous web proceeding from a dryer unit of a papermaking machine, the web having a moisture content below about 8 wt. % and a temperature of at least about 80xc2x0 C., which comprises conditioning the web by applying a flow of moistened gas having a temperature in the range of from about 10xc2x0 to about 65xc2x0 C. to a surface of the moving web across its width and along at least a portion of its length from a plurality of substantially overlapping flow zones wherein the flow is sufficient to create a combination of suction and pressure forces on the surface of the web to enhance convective heat transfer and thereby decrease its temperature. Depending on the amount of water applied to the web via the moistened gas, the conditioning may, in addition to decreasing the temperature of the web, increase the moisture content of the web. After the conditioning treatment, the web may be further treated in a process such as calendering, coating and the like. If desired, the web surface, after conditioning may be further moistened using a steaming device.
In a preferred embodiment, moistened gas is applied to the web in the aforementioned manner of overlapping flow zones using one or more arrays of radial jet reattachment nozzles. The nozzles are configured and spaced relative to each other and to the surface of the web to cause the moistened gas to be delivered relatively evenly across the web surface in flow patterns which create a combination of suction and pressure forces on the web. This enables the desired rapid surface cooling and moisturizing effect on the web as it proceeds from the dryer unit to any subsequent steaming and/or calendering steps.
In another aspect, the invention provides a papermaking process which comprises depositing an aqueous slurry of cellulosic and/or synthetic fibers at a consistency of from about 0.2 to about 1.5% by weight on a moving web former screen thereby forming a layer of slurry on the screen. The slurry is dewatered on the moving screen to form a fibrous web which is passed from the screen and then pressed with one or more wet press nips to provide a pressed web having a solids content in the range of from about 32 to about 45% by weight. The pressed web is then dried such as on a series of drying cylinders or other suitable drying equipment to provide a dried web having a moisture content of from about 0.2 to about 6% by weight. Thereafter, the dried web is conditioned by applying a flow of moistened gas to the web surface using a plurality of radial jet reattachment nozzles placed in close proximity to the web on one or both sides of the web to provide a conditioned web having a moisture content which is substantially uniformly increased across its width by at least about 0.2% relative to the moisture content prior to conditioning. The conditioned web may then be further rewet, if desired, by steaming or other means, and subsequently smoothed in a calendering unit or such operation. Alternatively, the conditioned web may be coated, which conditioning provides improved coating holdout.
One advantage of treating a web on a papermaking machine according to the invention is that the web may be uniformly and efficiently moisturized and cooled substantially below the temperature of the dried web proceeding from the dryer unit, preferably reducing the surface temperature to less than about 80xc2x0 C. using an even application of moistened gas so that any subsequent rewetting of the web occurs in the absence of deleterious effects associated with rewetting higher temperature product before calendering. For example, adverse effects on the cross directional shrinkage of the paper or paperboard product may be limited and desired density, tensile strength compression and caliper in the cross machine direction of the finished product may be achieved more readily and consistently with improved control over these and other properties of the finished product.
Another advantage of conditioning a web according to the invention in the papermaking context is that the resulting web thickness and stiffness after calendering to a desired smoothness may be improved as compared to webs conditioned using conventional techniques. Accordingly, the paperboard product can be made with increased bulk for a given basis weight and a product having a reduced basis weight will still meet caliper specifications. The more efficient surface cooling and moisturizing of the web obtained by use of the invention also enables increased spring back properties during calendering since the moisture is retained by the surface fibers of the web more efficiently than with other moisturizing techniques.
In some applications, it may be desirable to cool the web surface without significantly increasing the moisture content of the web. To this end, it will be appreciated that the invention may be practiced to cause emanation of a mist of minute water droplets from an array of radial jet reattachment nozzles wherein the water droplets have sufficient momentum to penetrate the boundary layer of hot, dry air attached to the moving web so that they impact the web surface. The moisture impacting the web surface rapidly cools the surface by acting as both a latent and a sensible heat sink. The applied surface moisture flashes to vapor upon contact with the hot web, thereby cooling the web. By use of an appropriate amount of moisture in the gas, the web surface is cooled without significantly increasing the moisture content of the web. Very high shear rates are attainable using the reattachment nozzles in a reattachment zone of the nozzle flow pattern which provides high convective heat transfer and high mass transfer coefficients to effectively xe2x80x9cscrubxe2x80x9d the web surface resulting in more efficient heat transfer from the web surface.
In applications requiring both cooling and moisturizing, higher mist loadings may be applied to the web surface with the reattachment nozzles resulting in substantial retention of moisture on the web surface. Hence, the web is both cooled and moisturized. This limits or avoids entirely the need to apply moisture to the web using conventional water spray nozzles or other means. If additional surface moisture application is desired, existing methods of applying surface moisture become more effective because of the cooling effects provided by this invention. Furthermore, the reattachment nozzles have fewer moving parts than water spray nozzles thereby reducing the maintenance costs associated with cooling and/or moisturizing a web.